Voltage snubber circuits are generally employed in control applications where a transistor is required to interrupt the supply of current to an inductive load. The voltage transient VL at the instant of current interruption is given by the expression: EQU V.sub.L =L * (di.sub.L /dt) (1)
where L is the inductance of the load and (di.sub.L /dt) is the rate of change in the load current i.sub.L.
Snubber circuits characterized by the circuit of this invention limit the transient voltage V.sub.L by controlling the turnoff rate of the transistor. A conventional circuit of this type is depicted in FIG. 1, in the environment of an automotive fuel injector driver circuit. In that figure, the circuit 10 is employed to bias the MOSFET 12 on and off to periodically energize the solenoid coil 14 of a conventional fuel injector. The automotive ignition voltage (IGN) is supplied to the unswitched terminal of the coil 14, and a second voltage (2V.sub.cc) is provided for biasing the MOSFET conductive when connected to its gate (g) via closure of the switch 18. The capacitors 32 and 34 are internal to the MOSFET 12 and represent the inherent gate-to-drain (C.sub.gd) and gate-to-source (C.sub.gs) capacitances, respectively. The remaining circuit elements--diode 20, Zener diode 22, resistor 24, capacitor 26 and current source 28--form the voltage snubber. The snubber circuit is activated at turnoff of the MOSFET 12 by opening switch 18 and closing switch 30.
When switch 18 is closed and switch 30 is open, the gate g is raised substantially to 2V.sub.cc and the MOSFET 12 is biased conductive in its drain-to-source circuit. In this mode, the diode 20 is reverse biased to block control signal loss through Zener diode 22, resistor 24 and the drain-to-source circuit of MOSFET 12.
When switch 18 is opened and switch 30 is closed to interrupt the conduction of MOSFET 12, the current source 28 starts discharging the gate capacitance C.sub.gs, and the drain-to-source voltage V.sub.ds rises abruptly in relation to expression (1) as the MOSFET 12 attempts to interrupt the coil current i.sub.L The rise in the drain voltage V.sub.ds forward biases diode 20 and quickly exceeds the breakdown voltage of Zener diode 22. At such time, the Zener diode 22 conducts current to the gate g via the limiting resistor 24 to maintain a partial conduction of the MOSFET 12. This serves to reduce the drain voltage V.sub.d by reducing the rate of change of current through the coil 14. Eventually, the drain voltage V.sub.ds falls below the breakdown voltage of Zener diode 22, and the current source 28 completes the turnoff of MOSFET 12. The capacitor 26 is required to prevent rapid changes in the gate voltage V.sub.g and the instability which would otherwise occur.
Although relatively simple, the voltage snubber circuit described above has some drawbacks. Most significantly, the capacitance (capacitor 26) required for stability prolongs the turn-on and turn-off times. This limits the time response of the circuit and is a serious drawback in an application such as a fuel injector driver, where fast speed of response is important. Moreover, the cost of the external components 20, 22, 24 and 26, as well as the circuit board space required for their assembly, is substantial.